1. Field of the Invention
It has recently been shown by Ilenda et al. in U.S. Pat. 4,957,974, incorporated herein by reference, that segmented copolymers, such as graft copolymers, of polyolefins, such as polypropylene, within a specific molecular weight range, and of polymethacrylates within a certain composition and molecular weight range, are useful additives for polyolefins for imparting melt strength. Such segmented copolymers are also useful for the compatibilization of polyolefins and polar polymers. An improved process for the manufacture of these segmented copolymers has been sought to lower cost of manufacture and to yield the product in a more suitable particulate form free of agglomerates and fines.
2. Description of the Prior Art
The term "segmented copolymer" refers to polymers wherein at least one segment of polymer A is chemically joined to at least one segment of different polymer B, and encompasses block copolymers, where the segments are joined at at least one end of the segments, and also graft copolymers, where there may be a trunk of polymer A to which at least one segment of polymer B is attached at a site on the trunk which is not at the end. Because it is difficult cleanly to separate and analyze polymers where a vinyl monomer such as styrene or methyl methacrylate is polymerized in the presence of a crystalline polyolefin, such as polypropylene, and because the possibility exists for both block and graft copolymers to be formed, we have chosen to use the inclusive term "segmented copolymers".
The prior art discloses many grafted polymers from vinyl monomers onto pre-formed polyolefins, which art is noted in the Natoli et al. application discussed below. The prior art further discloses many technologies to prepare segmented copolymers, especially by the polymerization of vinyl monomers in the presence of pre-formed polyolefins, such as in solution, in emulsion, in a solvent-swollen aqueous dispersion, and in an aqueous dispersion without a solvent. Again, relatively few of these references relate to aqueous-related processes wherein the vinyl monomer is an ester of a lower alkyl (meth)acrylate, and none disclose the specific polymers with high molecular weight grafted chains as taught by Ilenda et al. The methods disclosed for the polymerization of methacrylate ester monomers, such as methyl methacrylate, in the presence of a crystalline polymer, such as polypropylene, utilize methods which require a period of contact between the polymer and the monomer to be polymerized, which may further require the use of a solvent. The art does not teach a rapid method for conducting the polymerization, and such a rapid method is desirable for commercial production.
A major difficulty with such rapid methods is achieving penetration of the crystalline polymer particle by the monomer, which is best effected by a co-solvent which is essentially inert to the free-radical polymerization process, and further by the use of the polyolefin in the form of flakes, pellets, or porous spherical particles. An advantage of the present process is that it is as effective in producing acceptable segmented copolymer, of good appearance and physical properties, with less expensive pellets as with the more expensive porous particles. A second major difficulty is the tendency of the polyolefin to agglomerate or clump together upon contact with the monomer/solvent mixture during the initial stages of polymerization. Dispersing agents have been taught for the older processes known to the art, but these are ineffective in the present process.
One method involving a "slurry" polymerization of polypropylene in an aqueous media with methyl methacrylate or other methacrylate-rich monomer mixtures, appropriate organic solvents for the methacrylate monomer, and selected dispersing agents has been revealed in Natoli and Chang, U.S. application Ser. No. 898,979, filed Jun. 15, 1992, allowed, and incorporated by reference. This method has proven to be an effective means of manufacture. However, it suffers from certain process deficiencies such as agglomeration of the particulate structures when higher solids levels are attempted or when the second segment (the segment derived from at least 50 weight percent of an alkyl (meth)acrylate) is a polymer which softens at a lower temperature than poly(methyl methacrylate).
Thus, the art does not teach how to accomplish the desirable goal of a rapid process for forming in an aqueous medium at high solids without agglomeration the graft copolymers of the compositions discovered by Ilenda et al. The process is further useful for preparing related segmented copolymers wherein the molecular weight of one or more segments is lower than or higher than that taught by Ilenda et al. or where the composition of the (meth)acrylate group differs somewhat in composition from that taught by Ilenda et al.